An image forming apparatus, such as a copying machine, brings an exposure lamp into a reciprocating motion along with a document platen to scan a document on the document platen by the light from the exposure lamp in the reciprocation. The reflected light from the document obtained though the exposure scanning is irradiated to a charge coupled device (CCD) sensor that is a photoelectric conversion element via an optical lens. The CCD sensor receives the reflected light form the document onto a light receiving face thereof to make line scanning on the light receiving face then outputs an image signal with a level corresponding to a light receiving quantity. Thus, the image on the document is optically read, and the read image is formed on a copying paper.
The CCD sensor causes a problem of variance of sensitivity for each CCD sensor, or of unevenness due to changes with age. The brightness of the exposure lamp is also uneven for each lamp, or has unevenness resulted from changes with age.
Therefore, such a copying machine performs exposure-scanning of a white reference board which has been prepared in advance before exposure-scanning to read the document. The copying machine stores the output from the CCD sensor obtained through the exposure-scanning as a white reference value, and the machine corrects the output from the CCD sensor for exposure-scanning to read the document after that time in accordance with the obtained white reference value. This correction, a so-called shading correction eliminates spots of the read image due to the unevenness of the CCD lamp, the exposure lamp, the optical lens, etc.
However, granularity of the surface of the white reference board, dust attached thereto, dirt attached thereto, or the like produce the problem such that stripe-like noise appears in the read image.